Melting point bar



Jan. 17, 1956 W. F. BRUCE ET AL MELTING POINT BAR 2 Sheets-Sheet l Filed May 22, 1952 5M@ www w @if o o w m N\ R ww Q L Tp; WHW i rl WS N MI W lldrihl n? 131%@ WH www@ WUMIMIW ,E UNH! f .W n?. y wmwwmfmwnfwwwwwwwwwmw y Wmv. t n.1. A. L n G uw N Nw um@ E w m w. n, E v A S M was .E A A @n Q. ww Al Jan. 17, 1956 w. F. BRUCE ETAL MELTING POINT BAR Filed May 22, 1952 2 Sheets-Sheet 2 tirely.

shielding it is not possible to secure au apparatus which, in daily operation, will consistently give a specic temperature, with negligible variation, at the same place on the heated element.

,InY an effort to avoid some of these difficulties, and to provide an apparatus in which the temperature of4 the heated element would vary from the hot end of the heated bar to its cool end in an approximately linear relationship, so that the temperature at any point on the bar could readily be estimated, equal increments of distance corresponding to equal differences in temperature, an improved apparatus of the heated bar or hot bench type was developed. In place of using the generally unsatisfactory thermocouple of the earlier construction in which a constantan element slidable along the surface of the heated bar was pressed into contact with that bar at the point at which the substance melted, the improved form of apparatus omitted the thermocouple arrangement en- Instead, the device was calibrated by determining points of `known temperature by the use of substances of known melting points placed on the heated surface, these points of known temperature then becoming the key points from which temperatures at other portions of the heated element could be estimated. While avoiding the erratic results and inaccuracy characteristic of previous devices when employed under the circumstances described, calibration of the apparatus by means of substances of known melting point has not been entirely satsifactory, since it necessitates the additional labor of melting these substances of known melting point and determining these points. If only a single calibration were necessary this would not be so objectionable, but it is necessary for the operator in order to obtain reasonably accurate results to calibrate the device at regular intervals, especially each time that changes occur in the temperature conditions under which the determinations are carried out.

In providing a metallic element heated at one end in which the temperature differentials from the heated end to the cool end of the bar are substantially uniform for equal increments of length along the bar, it has been necessary to form this metallic element out of two different metals whieh are in contact with each other. The contact of dissimilar metals sets up an electrolytic couple vwhich accelerates corrosion of the electrically more active metal. In fact corrosion dculties with this type of hot bench are considerable, and they are increased by the manner in which the two-metal'heated element is heated at its hot end. Owing to the considerable amount of corrosion and to other diiculties, the device has not been generally accurate for temperatures in excess of about 250 C., and, in fact, use of the apparatus for the determination of temperatures above 250 C. is not recommended by the manufacturer.

While the idea of providing a heated element with a substantially linear temperature gradient is undoubtedly advantageous, satisfactory operation has not lat all times been possible, and it has not been always true that the temperature differential between points spaced equidistantly from each other along the hot bench was the same, regardless of their position on the surface of the heated element. This variability is, in part, the result of the failure of the apparatus to include any ,provision for avoiding sudden changes in temperature resulting from yarlations in the amount of heat radiated, or from changes in the temperature of the ambient atmosphere in the room m which the apparatus is used. This diculty, coupled with the necessity for repeatedly Calibrating the apparatus byl the use of substances of known melting points, there being no means on the device for accurately measurmg the temperature at any point along the heated element at points in between the calibrated points, has greatgtreduced the usefulness of the hot bench type of appa- In order to obviate these and other difficulties inherent Y duction.

4 i' in presently-available forms of apparatus for determining melting points and decomposition temperatures, and, in particular, to provide a form of apparatus of the heated bar type, similar in its more general aspects to the earlier devices discussed, but which apparatus will avoid the disadvantageous of the heated bar types of .device which are now available, we have provided the herein described apparatus.

VIn place of the two-metal heated element of the hot bench form of apparatus, with its considerable corrosion especially at the higher temperatures, we have provided a heated element formed of a single metal, the upper surface of this heated element providing the surface, or hot bar on which the substances undergoing examination are placed and melted or decomposed. We have, moreover, provided a shaped metallic element which is heated, this bar being so shaped as to secure an approximately linear temperature gradient on the operating surface thereof so that, for equal length increments along the surface of the heated element, equal differentials in temperature exist. This is accomplished, not by the use of an objectionable two-metallic system wherein the heat is conducted along the heated element by two metals of different heat conductivities as in the hot bench type of apparatus, but by shaping the single metal heated element (or bar) so that heat is conducted along the shaped bar by a progressively smaller mass of metal, thereby insuring the desired approximately linear temperature gradient on its operating surface. By thus utilizing a progressivelyV limited heat conduction, in place of depending on heat losses by radiation to regulate the temperature of the heated element so as to provide an approximately linear temperature gradient, we have practically applied the Stephan- Boltzman law of heat radiation to a problem of heat con- Radiation losses, in our form of improved apparatus, can thus be minimized and kept constant by the provision lof heat insulating means, since the distribution of temperature along the bar in our apparatus is regulated by control of the heat conduction by properly shaping the heated element, and no reliance need be placed 0n heat losses occurring as a result of radiation from lthe heated element.

ln place of the single thermocouple of earlier forms of apparatus as constituted by the heated element itself and a constanten element adapted to be pressed into contact with the heated element at various points along its surface, we

have provided a plurality of approximately equidistantiyspaced thermocouples which are permanently positioned,

prefer to use a series of thermocouples, all electrically connected through a single potentiometer for observing the indicated temperature. In this way the necessity for calibration by melting substances of known melting point on the operating surface of the heated element is avoided, and the temperature at any point adjacent a thermocouple can be accurately measured, the temperature at points in between thermocouple locations being accurately estimated by-measuring the distances to points of known temperature, since the temperature gradient along the operating surface is substantially linear.

Since radiation of heat forms a minor part of the operation of our improved apparatus (as it does in the operation of older forms of heated bar apparatus) it is evident that we may effectively shield our heated element so that inaccuracy dueto variations in the amount of heat radiated, or in the temperature of the atmosphere in the room in which the device is utilized, will not occur. In this way erratic results and inaccuracies in the determination of temperature are'avoided'.

.arranges ..As.difliculties,.astthe resultrof;` corrosion, iirst become importantA `at .thea-heated. end Ifthis `type. of heated "bar or hot bench apparatus, we have providdran (improved "electrical :heating...means, so designed1 asgreatly to reduce corrosion .anda perrmit.the-usegoffthe .device in f Itis, accordingly, one of ther-objects ofrhis invention to provide a new and improved apparatus for determining the' melting points and. decomposition `temperatures of organic compounds `and `chemical vsubstances and mixtures, which `apparatus Will -bertusable over a` considerable range of temperatures,` andz-whichwillpermit the rapid and precise determination lof fthese physical contents for materials melting ordecomposing over a Wide range of temperatures.

It is another 'object `of our invention to provide melting point determination'apparatuswherein a heated metallic element is utilized, `thisheated. element `being preferably formed as a shaped. plateuorbarsand..providing an i operative surface on which thesubstance undergoing test may be placed, andi beingfurther :so proportioned as to i provide a substantially. linear .distribution i ofl temperatures over the operative surfacefthereofwithin a useful ternperature range.

lt is still another object of our invention to provide an apparatus for -the ydetermination fof meltinglpoints and/ or decomposition temperatures,` which apparatus utilizes a metallic element -which is heated lat one `end thereof, thisheated element being `of afsingle `.metal and soshap'ed as to provide a substantially uniform temperature gradient `.along points linearly spacedfromeach other on the operating `surface thereof, this "heated lmetallic lmember @being further provided with alplurality ofequidistantly fspaced,- permanently fixed, temperatureldetermining"devices which are `positioned so 'as 'to determine thei vtemperature' at various points alongthe 4operating surface of `the heated element, said 'devices being thermally in contact withtheheatedelement butJelectrically insulated therefrom.

A further object of our'invention is to provide melting point determination apparatus' of an improved construction wherein the' `heated metallic element'` 'on `which`fithese vsubstances undergoingtest are" melted "or decomposed is arranged with suitable shieldingmeans adapted to` reduce radiation therefrom tol a'minimum, whereby errors due to variation inthe amount 'of `heat radiated and changes in the temperature ofthe ambientatmosphere are. greatly reduced, thus permitting `the uniform securing of accuratevalues forthese constants of the substances undergoing test.

Still another object of our invention is to provide new` and improved heating means for heatingrthe metallic element from one endthereof, said improved heating means, together with the use of al singlemetal for `the heated metallic element, permitting `heating toIhgher temperatures without excessive corrosion,`.therebyA providing for use of the apparatus `to determine temperatures within the very useful `elevated range (especially-as regards decomposition), up to 350 .to `4O()""C.

Still a further object of our invention is to providean Aimproved apparatus for the determination of these critical constants, i. e. melting points and decompositiontemperatures, wherein the'operatingsurface ofthe heated'element, above the temperature'determining orrecording devices, is provided with a graduated scale lwhich permits ascertaining 'theetemperature at `'anyjpoint on'L `the operatas in pyramidal or frustofpyramidal. shape. Other .con-

. ing surface at a` point in between those at which the teniperature is determined or' recorded by said devices.

The foregoing objects, as .vell'as additional objectives of our invention, will be e'vident'from the ensuing disclosure of a preferred embodiment thereof.

This apparatus is best understood in connection with the annexed drawings, wherein:

Fig. l is a View in side elevation of .the entire apparatus Aas .enclosed in a protective casing,` the view `including the potentiometer shown as separate fromthe apparatus;

. 2 is an elevational View of the apparatus as seen with one of the sides of lthe enclosing casing removechthe `left-hand endY of `ithe heated element. and certain portions i ofthe casing and its supporting brackets, for convenience,

being illustrated in section;

Fig. 3 is a viewin elevation of the heated metallic ele- .srnent showing the apertures in one sidethereof in which the temperature indicating devices are positioned;

`Fig. 4 is atop view of theheated element, this View `-showingthe Pgraduation marks on the'upper edge portion tot operating' surface thereof;

5 is a longitudinal.cross-sectional view of the heated lelementtaken.substantiailyon the line 5 5 of Fig. 6;

Fig. 6 is anend view fof the fheated element at its end of greatest depth, this VView showing the sockets in which the electrical heating elementsfare` adapted tobe positioned, out with those heating elements removed;

t Fig. 71is `an enlarged'viewfofparttoftheiheated element :showingfone ofthe thermocouple 'temperature-indicating :devices positioned .inone of theapertures provided therec-lorain the side portion of the metallic element, adjacent :the operating surface thereof;

f Fig. 8 is awiring ldiagram showing .the connections to :the main power lineand the control switches by which the electrical heating unitsifor heating the heated element may be turned on or off, as desired, either singly, or together;

Fig.-9^is-a wiring-diagram showingfthe connections to Athern'ainpower linegand 'the electrical `conduits by means -of which the individual'` thermocouples maybe connected toa potentiometer` through a suitable tap switch having a plurality of contact pointsfrwhereby the temperature `may be; determined at. any ofthepoints alongrhe heated ele- -lment at lwhichtherrnocouples are positioned;

`Fig. `l() is -a transverse..crossfsectional view` showing the heated elementand theisur-rounding protective casing, this view beingtalten on the `line lil- 10` of Fig. l; and

A 'Fig llmis adiagramrnatic representation wshowing `the 50 .shape of y,the curve of `the lower portion of the plate `when shape being'sowdesignedias -to provide `for a substantially an aluminum plate-isutilized -for `the. heated element, :its

linearidistributionaof.-y temperature `over the operating surface of-the-heatedelement. `This `diagrammatic representation shows the ordinate .distances `as measuredtdown- 55,:

'the .-relativenumerical valuesof these ordinate distances,

wardlyat equalintervals from a horizontal base line, and

defining the shape of Ythe-.bottom curve, are `tabulated i below.

Our improved apparatus includesthe heated element 11', preferably formed of arelatively thin .aluminumfplate t This-'heated elementis heatedat one end thereof, and

its.uppernsurfacelZiconstitutes the operating` surfaceon f which the .substances .undergoing .test Vareplaced in .order `thatathey. may.y melt, ror=may decompose without melting.

`For this purpose, ifdesired, `the operatingsurface 12. may

. beslightlywtroughed transversely, or 'it` maybe a substantially plane surface.

. Insteadmf constructing the heated element 11 in the formof. akthin aluminum plate, .it may be formed `as a block of metal of a greater thickness, such, for example,

ve'nient forms may be utilized', but for most purposes we have" found thethinjplate form to be most satisfactory forthe heatedfelement 11.

The lower edge portion of heated element `111is curved as shown at 14, see particularly Figs. 2, 3, and 1l. .The precise form that this curve may take for a relatively thin plate of aluminum or aluminum alloy is more fully explained hereinafter. It is neeessary that themass of the plate 11 be so distributed with respect to its linear extent, from its heated end outwardly, that a substantially uniform temperature gradient is maintained along the operating surface 12. In other words, for substantially equidistant points along the operating surface 12, uniform differentials in temperature exist, regardless of whether the points are adjacent the hot end of the bar or adjacent the cooler end thereof, within the limits of the length of the particular portion of the heated element which. is employed in the determinations of melting point or of the decomposition temperature.

The heated element 11 is therefore so shaped as to provide, per unit of length, a progressively smaller mass of metal proceeding from the hot end to the cool end thereof. Since the quantity of heat transmitted depends on the cross-sectional area of the heated element, the approximately linear temperature dierentials throughout the working extent of the heated element result from a progressively limited conduction of heat through the heated element or bar. Radiation of heat is minimized and held relatively constant by providing an insulating box or easing around the heated element, as subsequently explained.

The heated element 11 is so shaped by properly forming the curve 14 of its lower edge portion, in accordance with the Stephan-Boltzman law of heat radiation as applied to heat conduction, that the quantity of heat transmitted lengthwise of the heated element is such as to secure the approximately linear distribution of temperature therein. In order to secure this result radiation of heat from the heated element must be minimized and reduced.

Adjacent the upper or operating surface of the heated element 11, and just below this surface, there is provided a series of small apertures 15 which are equidistantly spaced between centers along substantially the entire useful length thereof. Each of these apertures is adapted to receive a temperature indicating device, which may be a small bulb thermometer or, preferably, an electrical thermocouple element. These apertures or cavities 15 are made as small as possible in order to interfere to a minimum with the conduction of heat through the heated element 11 to the sample and to other parts of this heated element. On the operating surface 12 immediately above each of these apertures there is inscribed, while the bar is heated and at operating temperature, a series of graduation marks, indicated by numeral 16. Since the distance between each. pair of adjacent apertures 15 is the same throughout the entire useful length of the heated element 11, graduation marks 16 are spaced at the same interval on the operating surface 12, each mark being preferably inscribed on the metal of the operating surface immediately over each of the apertures 15.

Merely as illustrative, and as in no sense limiting the scope of our invention, we may note that in one embodiment the apertures 15 have been spaced from each other at a distance of 20 millimeters between centers. The individual graduation marks 16 on the calibrated operating surface 12 have therefore also been spaced at a distance of 20 millimeters from each other. The heated element has been so constructed by shaping the curve 14 of its lower edge portion that the temperature differential between any two adjacent points, as marked by adjacent calibration marks on the operating surface 12 of the bar, is 20 C. when the heated element is heated by the electrieal resistance heating coils. Since the graduation marl-:s are 20 millimeters apart, a distance of one millimeter along the operating surface represents a temperature differential between the two points spaced from each other by this distance of l C., since the distribution of temperature along the operating surface of the heated element 11 drops substantially uniformly, or as a linear function, along the operating surface from the hot to the cool end thereof. v V

There may, of course, be anynumber of individual apertures 15 each'designed'to receive a thermoeouple or other temperature indicating device, and for convenience we have illustrated a construction in which there are twenty-three such apertures, see particularly Figs. 2 and 3. Of course the distance between thermocouples may be varied as desired, and the number of individual therinoeouples, or other temperature indicating devices, provided along the heated element 11 could be made greater, or less, as desired, depending on convenience and on the operating temperature range through which the melting point determination apparatus is designed for operation.

At the left-hand or heated end 18 of the heated element 11 (as viewed in Figs. 2 to 5) the thickness of the plate is increased by bolting thereto on either side thereof, two lateral blocks or plates 20, 21. The plates 20, 21 are secured to the main body of the heated plate element 11 by means of heated bolts 22 and 23, and they serve to enlarge the thickness of the heated element at the end thereof, 18, at which the heating elements are to be positioned. The end of the enlarged structure thus resulting is bored out as shown in Figs. 2, 5 and 6 to provide bore holes 24, each of which is adapted to receive an electrical heating element (Fig. 2). These heating elements may be ordinary soldering iron heaters having an electrical heating coil, preferably enclosed in brass casings, and in one embodiment of our invention we have successfully used soldering iron heating coils of about 300 watts power.

In assembling the melting point determination apparatus it is preferable to lubricate the electrical heaters 25 after they are inserted in place in the apertures 24 by means of a silicone uid, preferably a silicone uid of relatively high boiling point, in order to minimize corrosion at this portion of the apparatus. In place of electrical heating elements, heating by some other means, as by means of a gas flame, could also be utilized, but we much prefer to utilize electrical resistance heating elements of the type illustrated and described.

Electrical power to each of the electrical heating elements 25 may be controlled by means of a separate switch, the manually manipulated portions of these switches protruding through the easing 26 which surrounds the heated element 11 and the electrical. conduits. In the embodiment shown these switches extend through the casing 26 at that end of the apparatus which is remote from the end in which the heating elements 25 are positioned. As shown in Fig. 2 and schematically in the wiring diagram Fig. 8, electrical conduits, designated as 28, extend bctween the heaters 25 and the individual switches for controlling the heaters bearing numerals 31, 32, 33 and 34. Plug 35 in the circuit is adapted to be plugged into a suitable souree of power, such as ordinary -115 volt alternating current. By this arrangement it is possible to turn off one or more of the heating elements 25 as desired, and to have any number of heating elements, up to four operating at the same time.

ln one embodiment of our invention we have found that by utilizing soldering iron heaters of 300 watts, when the device is operated with one heater 25, a maximum temperature of about 300 C. is attained at the hot end 18 of the heated element 11. Two heaters will providea temperature of 400 C. at the hot end, which is ordinarily as highla temperature as it is desirable to maintain at this end since most substances will either melt or decompose at temperatures of below 400 C. it is possible, of course, to secure higher temperatures by turning on three or even four heaters, but ordinarily we prefer to operate with two heaters turned on at one time, two of the electrical heaters 25 being held in reserve. In this way, it is possible to maintain any temperature, as

t trated to a23-point `selective :tapiswliteh eamsas desired, -and since replacement unitsl are c available for `heating or for interchange, rinterruptions inservice `due to failure ofthe electricalrhsating means are extremely unlikely.

It is` also `possible' to operate so that all four heating `elements are controlled byone switch, the "circuit including a variable'transformer. lnthis way, the-temperature at each individual heater .'itneed not be auythigher than that needed .to achieve thetdesired heat input,;the load lof heating `being divided between four `individual ele'ctricaltheaters `25, all of `which arentilized at one time,

`and all being controlled from `one switch and by means `insulated fromthe-heated `elernenthy `means of `insolute `ceme`ntf39. Thermocouplcs comprising other dissimilar metals may also be utilized, but rwe have found chromclt alumel `thermocmtplesto `giveyery satisfactory results. 4Ordinary thermometers `of a sufliciently small size, such t as small bulb thermometersycould alsov be used in `the apertures l inV place *of thernrocouples, but ordinarily there is` no particular advantage in this.

`As shown `in Fig; 2, andin the wiring dia-gram Fig. 9,

t the aiumel thermocouple vterminals all lead bymeans of connecting wires 4l toacommon conductor Il?. which is `All i `secured to one terminalii of potentiometer` de. conduits 41,42 are of zalurnel. i Chromel terminals 37 `of the thermo'couplestare.eachfprovidedvwith a separate `lead `frmneach `of vthe twenty-three `thermocouples illus in Fig?. each of these wiresfrom.individual;ohromelzthermocouple terminals is designated by the same numeral 47". "For convenience, `allx23 ofthe vL'ires'd'1 maybe united into a single bundle `of wires 113,'.1this bundle `kt-:eping the wires together Vinside of the `casing :26 which surrounds heated element Til and electrical conductorsZS, 4i, 42, 47 andati.v

Theselective `tap` switch 45, whichin 'the` embodiment illustrated` is provided `with 23rpoints although, of course, it may have a fewerorgreater number thereof, isffot' rotary type and permitsclosingfthe circuit through. the potentiometer dii-,this"circuitlincluding any one of `the chromel thermocouple terminals and'its corresponding alumel terminal. Lead S front the rotary larm 57 'of the tap switch Gldsconhectedaby aconduit to the other terminal 52 ofwthe `potentiometer144. Toggle switch S4 extendsacrossA the circuit `between thecommon alumel conduit i2 `and leadSLithis switch being in circuit` with resistance''whi'ch,in atypical case,` may oe of 70 ohms. `Electrical"conduitsd2 andblt lead to potentiometer' 44 which maytconvenie'ntly be a` nrillivoltmeter.

When the toggle switch 54 is closed the-potentiometer 44 `will indicate a diiference infp'otential in `rriiilivolts `which is proportional Atothe temperature iat that point along the heated elementiiliatl whichUrbe-particular thermocouple, selected by means offrotarytap switch t 45, is positioned. The particular therrnocouple point in `compressed asbestostproduch iHov/everfother nonconducting materials such as bakelite could also he used, and itis possible `to employ most any nonsoftening, thermallyresistant plastic, or even a natural materiaL'such as wood. While metal could also be utilizedv for the casing`26, ordinarily we prefer to utilize transite, since it is light, electrically and heat insulating, inexpensive, and capable of being readily cut to shape to provide an effective radiation Shield.

The casing 26 includes side, top, bottom and end panels, and serves to lessen losses of heat due to radiation, `and to protect the heated element 11 from drafts and air currents which would result in uneven temperatures along the operating surface 12 thereof. The panels forming the housing 26 are supported by avplurality of metal brackets 61. Two separate strips 62 (Fig. l0) supported by metal brackets 61 `are `positioned on either side of the heated element ill, and they form the top of the housing or casing 26 at this point, but they are so constructed as to provide between them a limited area of access to the operating surface 12. This arearisrso limited in extent that losses of heat as a result of radiation therethrough are not important.

The side panels 50 of transite or similar materal comprising the casing 26 are secured by means oftbolts 63 totbrackets 61 (Fig. l), and bent metalstrips 65 support `the .endtpanels 66 in sucha way that they may be readily removed in order to gain access to the interior of the housing and to the electrical resistance heating elements 25. The four switches 31, 32, 33` and 34, which are used `for turning on `and off current to the heating elements 25, protrude through side panel Si) l.of the casing adjacent the bottomthereof, while the handle? 68 controlling the rotary arm 57 of rotary tap. switch 4S protrudes'through an upper panel of the casing at that end remote from the heating element 25. The .toggle` switch 54, `con- 'trollingrthe circuit which` includespotentiometer 44, 1also `protrudes through the casing `at this end, so that it is readily available to the operator.

In the diagrammatic respresentationshowing the shape of the curve 14 for `the lower edge portion ofthe heated element 11 `whenwathin aluminum plate `isutilized (Fig. l1), fifteen points are indicated `asabscissas along `the upper` oroperating surface 12, while the ordinate -distances measured downwardly at each of these .fifteen `points to the curvededge 14 isindicated by'the letter d. Point l is at the hot end of the operating surface 12,

iimmediatelyovertthe point-at which curved edge 14 joins the `hot end portion 11S-containing the `heaters 25, while point l5 is the last useful pointon-thetoperating surface at the cool end of the bar.

The ordinates of cui-veld` for each of these 15 points are given inmillimeters below,.tit `being understood, of

- course, thatthis shape for the curve 14 islimited to a heated element in the form `ofna `thin aluminum` plate, each point from 1 to 15'ftbeing equidistantly spaced from adjacent points on eitherside by a distance of 2O millimeters.

Pointz Distance from operating surface, mms.

l 6G 2 48 3 l1 4 34 5 27 6 2l 7 16 8 11.5 9 8.5 l0 6.5

'(Pointsl are 20 .millimeters aparhameasuredthorizontally.)

In using the apparatus to determine the melting point of the substances, or the decomposition temperature in the case yof those substances which decompose before they melt, a small amount of the material to be tested is moved along the operating surface 12 of the heated element 11 until a point is found thereon at which the substance either just begins to melt, or just begins to decompose. lf this point happens to fall on one of the graduations 16 on the operating surface 12 at which point the temperature is measured by one of the thermocouples, it is of course possible by selecting the proper circuit by means of the rotary tap switch 45, to measure the temperature at this point, the value being directly indicated on the potentiometer 44.` ln this case there is a direct value obtained for the melting point of decomposition ternperature. On the other hand, and as moreV usually happens, if the point at which incipient melting or decomposition occurs is located between graduation marks on the operating surface i2., the temperature at this point is determined by interpolation between temperatures as measured at the adjacent graduation marks at which thermocouples are located, since the temperature gradient along the operating surface is substantially linear.

As an example, if the distance between graduation marks is 20 millimeters, and the temperature differential as measured by two adjacent thermocouples, one on either side -of the point on the operating surface 12 at which the material melts, is 20 C., ythe melting point is readily determined simply by measuring the distance in millimeters from the point at which the substance melts (or decomposes) to the nearest adjacent graduation mark, each millimeter, in this illustrative case, representing a temperature gradient of 1 C.

When using our apparatus to determine the melting point or decomposition temperature of a material which might corrode the aluminum or aluminum-alloy out of which the heated element is formed, or which might conceivably alloy with the metal of this element along the operating surface thereof, we generally prefer to place the material to be tested on the thin foil of resistant metal, such as silver, tantalum or platinum. In this way, corrosion and alloying are avoided, without in any way changing the manner in which our apparatus is used.

Aso, while we have shown a rotary tap switch 45 with twenty-three contact points it is not of course necessary to have any particular number of thermocouples, or any particular number of contact points on the rotary tap switch. It is only necessary that the distance along the heated element between points of known temperature should not be too great, and ordinarily we prefer to place the temperature determining elements 36 close enough together so that the temperature at any point between those at which the temperature is measured can be known with a fair degree of certainty by interpolation. For some purposes the thermocouples 36 can be spaced further apart than for other purposes, and the spacing will ordinarily be determined by the accuracy with which it is desired to estimate temperatures along the bar by interpolation between known temperatures as measured by the thermocouples.

We claim:

l. Melting point determination apparatus which comprises a longitudinally-extending heated element provided with an operating surface on which substances to be melted may be placed, said operating surface being shielded to reduce radiation losses, and having a substantially uniform temperature gradient for substantially equal length intervals along said operating surface so that the temperature varies by substantially equal increments between points at substantially equidistant intervals from each other along said operatingV surface, a plurality of electrical thermocouples positioned in contact with said heated element at substantially equidistant points below the operating surface thereof and adapted 12 to measure the temperatures along said operating surface at a plurality of points which are spaced at substantially equidistant intervals from next adjacent points, a single potentiometer for indicating the temperatures asy determined by said thermocouples, electrical circuit means electrically connecting said thermocouples and said potentiometer, and means for heating said heated element adjacent one end thereof.

2. Melting point determination apparatus comprising a longitudinally-extending heated element formed of a single metal and including an operating surface on which substances to be melted may be placed, said heated element being heated at one end thereof, and being so shaped, from its heatedvend outwardly, that said operating surface has a substantially uniform temperature gradient for substantially 'equal length intervals thereon when said heated element is heated, said temperature varying by substantially equal increments for points at substantially equidistant intervals from each other along said operating surface, a plurality of thcrmocouplesV positioned in contact with said longitudinally-extending heated element at substantially equidistant points therealong so as to determine the temperatures on said operating surface ata plurality of points along said opcrating surface which points are spaced at substantially equidistant intervals with respect to adjacent points, a single potentiometer for visually indicating said temperatures, electrical conduit means connecting said thermocouples and said potentiometer, a radiation shield substantially completely enclosing said longitudinallyextending heated element and positioned with respect thereto to protect said heated element from drafts and air currents which otherwise would result in uneven temperatures along said operating surface thereof, and means for heating said heated element adjacent said heated end thereof.

3. Melting point determination apparatus comprising a longitudinally-extending heated element formed of a single metal and including an operating surface on which substances to be melted may be placed, said heated element being heated at one end thereof, and being so shaped, from its heated end outwardly, that said operating surface has a substantially uniform temperature gradient for substantially equa] length intervals thereon when said heated element is heated, said temperature varying by substantially equal increments for points at substantially equidistant intervals from each other along said operating surface; a plurality of thermocouples positioned in equidistantly-spaced apertures provided in said longitudinally-extending heated element in proximity to the operating surface thereof so as to determine the temperature on said operating surface at a plurality of points therealong, which points are spaced at substantially equidistant intervals with respect to adjacent points; a potentiometer for visually indicating said temperatures; electrical conduit means connecting said thermocouples and said potentiometer; a radiation shield substantially completely inclosing said heated element but permitting access to said operating surface, said shield being effective to reduce radiation heat losses therefrom; and means including a plurality of electrical resistance heaters for heating said heated element at one end thereof.

4. Melting point determination apparatus comprising a longitudinally-extending heated element formed of a single metal and including an operating surface on which substances to be melted may be placed, said heated element being provided at one end thereof with a plurality of heater receiving cavities, and being so shaped, from its heated end outwardly, that said operating surface has a substantially uniform temperature gradient for substantially equal length intervals thereon when said heated clement is heated, Said temperature varying by substantially equal increments for points at substantially equidistant intervals from each other along said operating surface; a plurality of thermocouples positioned in equidistantly-spaced apertures provided in said longitudinallyt extending heated element in proximity to the operating surface thereof so as to determine the temperatures on said operating surface at a plurality of points therealong, which points are spaced at substantially equidistant intervals with respect to adjacent points; a potentiometer for visually indicating said temperatures; electrical conduit means connecting said thermocouples and said potentiometer; a radiation shield substantially completely inclosing said `heated element but permitting access to said operating surface, said shield being effective to reduce radiation heat losses therefrom; and means for heating said heated element at one end thereof, said means including a plurality of electrical resistance heaters positioned in said heater receiving cavities, electrical circuit means connecting said heaters to a source oi' electrical power, and means for controlling the supply of electrical power thereto whereby the temperature of said electrical heaters may be controlled.

5. Melting point determination apparatus comprising a longitudinelly-extending heated element formed of aluminum and including an operating surface on which substances to be melted may be placed, said heated element being provided at one end thereof with a plurality of heater receiving cavities, and being so shaped, from its heated end outwardly, that said operating surface has a substantially uniform temperature gradient for substantially equal length intervals thereon when said heated element is heated, said temperature varying by substantially equal increments for points at substantially equidistant intervals from each other along said operating surface, and said operating surface of said heated element being provided `with graduation marks thereon spaced substantially equidistantly along said surface; a a plurality of thermocouples positioned in equidistantlyspaced apertures provided in said longitudinally-extending heated element in proximity to the operating surface thereof so as to determine the temperatures on said operating surface at a plurality of points therealong, which points are spaced at substantially equidistant intervals with respect to adjacent points; a potentiometer for visually indicating said temperatures; electrical conduit means connecting said therrnocouples and said potentiometer; a radiation shield substantially completely inclosing said heated element but permitting access to said operating surface, said shield being effective to reduce radiation heat losses therefrom; and means for heating said heated element, said means including a plurality of electrical resistance heaters positioned in said heater receiving cavities, electrical circuit means connecting said heaters to a source of electrical power, and means for controlling the supply of electrical power supplied thereto whereby the temperature of said electrical heaters may be controlled.

6. Melting point determination apparatus comprising a longitudinally-extending heated element formed of a single metal and including an operating surface on which substances to be melted may be placed, said heated element being heated at one end thereof and being so shaped, from its heated end outwardly, that said operating surface has a substantially uniform temperature gradient for substantially equal length intervals therealong when said heated element is heated, said temperature varying by substantially equal increments between points at substantially equidistant intervals from each other along said operating surface, said heated element being in the form of a relatively thin aluminum plate with a substantially straight upper edge portion constituting said operating surface and a curvedlower edge portion whose curved outline is dened by the following ordinates in millimeters for points spaced equidistantly form each other at a distance of 20 millimeters as measured along said straight upper edge portion, said ordinates being measured from said straight upper edge and at right angles thereto.

References Cited in the file of this patent UNITED STATES PATENTS 2,015,838 Borden et al Oct. 1, 1935 2,336,238 Fordyce et al Dec. 7, 1943 FOREIGN PATENTS `815,706 Germany Oct. 4, 1951 

